This invention, which concerns the field of nonlinear optics, addresses two nonlinear optical phenomena: (1) the exchange of energy between two mutually coherent beams of light, and (2) the phase-conjugate backscattering of a single coherent beam of light.
On a fundamental level, nonlinear optics is the study of the interaction of light with matter. This interaction is nonlinear because incident light can change the index of refraction in some materials, thereby affecting the frequency, intensity, and/or the phase of the light itself. By providing a means to manipulate these properties of a laser beam, nonlinear optics has yielded new optical applications, including optical information processing, optical computing, optical filtering, laser beam control, and novel optical sensor designs.
Photorefractive two-beam coupling is an area of nonlinear optics that deals with the transfer of energy from one beam of light (known as the pump beam) to another beam of light (denoted the probe beam). This transfer of energy occurs with no phase crosstalk between the pump beam that is "donating" energy and the probe beam that is "accepting" energy.
The photorefractive effect is a nonlinear optical phenomena which occurs in photorefractive crystals, such as barium titanate (BaTiO.sub.3) and strontium barium niobate (SBN), and can be used to achieve two-beam coupling. Two mutually coherent laser beams illuminating a photorefractive crystal will cause an interference fringe pattern of light intensity to form within the crystal. This fringe pattern causes a charge separation, which creates an electric field that, in turn, induces a change in the index of refraction via the Pockels effect. The resulting volume refractive index grating (or real-time hologram) affects the propagation of the light beams in the crystal and allows the exchange of energy between the beams. This energy exchange by means of two-beam coupling lacks any phase crosstalk, i.e., one beam is amplified at the expense of the other, yet the spatial aberrations and frequency differences of the donor beam are not transferred to the acceptor beam. The discovery of this phenomena has led to a variety of new applications, including beam processing techniques, such as image amplification, laser beam cleanup, and beam combining, as well as device structures such as ring oscillators, laser radars, and sensor protection devices.
Photorefractive phase conjugation is an area of nonlinear optics that deals with the generation of a phase-conjugated beam of light. If a light beam is considered as the motion of a family of wave fronts in space, the phase-conjugate of that light beam has exactly the same set of wavefronts as the initial beam, but propagates in the opposite direction. This phase-conjugate beam is considered a time-reversed replica of the incident beam since it exactly retraces the path of the incident beam.
Typically, photorefractive two-beam coupling and photorefractive phase-conjugation require that the polarization of each incident beam be aligned in a specific direction with respect to the crystal to take advantage of the appropriate electro-optic coefficient. For example, in photorefractive barium titanate, it is common knowledge that the efficiency of two-beam coupling is maximized when the largest electro-optic coefficient r.sub.42 is employed which, in turn, dictates the extraordinary polarization requirements. Similarly, in photorefractive strontium-barium niobate, the electro-optic coefficient r.sub.33 is the largest and again extraordinary polarization is most commonly used. Restricting the incident polarizations to single polarization states can be accomplished with simple polarizing beam splitters by discarding the undesired polarization components of the incident beam; however, any nonlinear optical device based on such a scheme would be rendered ineffective for the wrong incident polarizations. Considerable advantage would be gained by devising a method of two-beam coupling which does not have such polarization requirements.
Competition from a phenomena called beam fanning imposes another limitation associated with photorefractive two-beam coupling and photorefractive phase conjugation. As a laser beam propagates through a photorefractive crystal, it scatters due to imperfections and impurities inside the crystal. The scattered beams interfere with the main beam and write their own photorefractive gratings, resulting in the scattered beams being amplified at the expense of the main beam. In two-beam coupling, this "noise" amplification consumes energy from the pump beam that would otherwise have been used for the amplification of the probe beam. In addition, these extra fanning gratings usually compete with the two-beam-coupling grating for the finite number of photorefractive charges which are available within the crystal, resulting in decreased two-beam-coupling efficiency. Similarly for photorefractive phase conjugation, these fanning gratings can compete with the phase-conjugate gratings, decreasing the phase-conjugate reflectivity. For these reasons, it would be desirable to be able to suppress this beam fanning phenomena while still maintaining efficient two-beam coupling or phase conjugation.